VEGETABLE CROPS

Clubroot of Crucifers

Fact
Sheet Page: 730.11 Date: 10-1985

COOPERATIVE EXTENSION 
NEW YORK STATE  CORNELL UNIVERSITY

by T. A. Zitter, Dept,
of Plant Pathology, Cornell University

Clubroot remains one of the important diseases affecting members
of the mustard, or Cruciferae, family. The disease occurs worldwide and affects
such important crops as cabbage, cauliflower, Brusselss sprouts, broccoli, and
Chinese cabbage. Every effort should be made to keep contaminated crucifer transplants
from entering a farming operation because the pathogen survives for many years
even in the absence of host crops. Clubroot can cause drastic yield reduction
and occasionally total losses in crucifers.

Cause

Clubroot of cabbage and related crucifers is caused by the soilborne
fungus Plasmodiophora brassicae. This relatively simple fungus forms
no mycelia (numerous threadlike branches) and is an obligate parasite, meaning
that it can develop and multiply only in living host cells. Its vegetative phase
consists of a multinucleate mass of protoplasm lacking a cell wall (plasmodium
or slime mold). Although the organism primarily attacks members of the Cruciferae,
the intensity of disease caused on different crops (see "Host Range") varies.
The organism also exists as many specialized forms or races, the variation in
plant resistance and ability to survive in different areas being explained thereby.

Symptoms

Clubroot may develop extensively on plant roots before the first
sign (an abnormal wilting and yellowing of leaves, especially on warm days)
is noticed aboveground (fig. 1). If the soil is moist,
these symptoms may not become apparent until water stress occurs. When infection
occurs at an early stage of growth, young plants are stunted and may die, whereas
plants infected in a later stage fail to make marketable heads or growth. When
diseased plants are pulled from the soil, the roots are usually swollen and
distorted (fig. 2). The name clubroot is derived from these
symptoms. Root malformation may vary in size from very small swellings on the
tap and lateral roots to large club-shaped roots, depending on when the plants
became infected. In addition to reducing the plant's ability to take up water,
the clubbed tissue fails to develop a protective outer layer and, thus, is susceptible
to invasion by soft rotting bacteria.

Click on Photo for Magnification

Disease Cycle

The disease cycle for clubroot is illustrated in fig.
3. P. brassicae is capable of surviving in the soil for 7-10 years
or longer as resting spores. The resting spores of the fungus can be spread
from field to field by infested soil, contaminated water supplies, infected
transplants, infested soil on farm machinery, and even by roving animals such
as cattle. When soil conditions dictate, the resting spores of the pathogen
germinate to produce zoospores, which are able to "swim" by means of flagella
to infect susceptible plant root hairs. The germination of resting spores requires
moist, acid soil and can occur over a wide temperature range of 54°-81
° F (12°27° C). Disease development is favored by high soil moisture
and soil temperatures between 64°-77° F (18°-25° C). Although
clubroot has been found in soils exhibiting a wide pH range from 4.5-8.1, the
disease is primarily associated with acid soils. Within the infected plant roots,
the organism develops rapidly, causing an increase in the number and size of
cells, which results in "clubbing." During the development of the organism in
the plant, new zoospores are produced; these are capable of infecting the same
plant or adjacent plants and, thus, repeating the cycle. Eventually, resting
spores are formed within the diseased plant tissue, and these are released into
the soil when the plant roots disintegrate.

Click on Drawing for Magnification

Host Range

The disease is restricted mainly to members of the mustard family,
both cultivated and weeds, and to a few other plants. Because of the existence
of races in this organism, plants can respond to attack on different levels,
but in the Northeast they can be grouped as follows:

Most susceptible:cabbage, Chinese cabbage, Brusselss sprouts,
some turnips, wormseed mustard, and some species of candytuft

Medium susceptible:kohlrabi, kale, cauliflower,
collards,broccoli, rutabaga, seakale, some turnips and radishes, and some
species of candytuft

Control Measures

Most control measures for clubroot are aimed at preventing the
introduction of the fungus into a field. Clubroot is not seedborne, but the
chief means of spread is by the use of contaminated transplants.

(1) Always use plants that are certified by the official state
certifying agency. Even if a trace of clubroot is present on a few plants,
reject the entire lot. Many other seedlings may be infected and are
simply not expressing symptoms.

(2) Carefully choose sites for crucifer seedbeds that do not
have a history of clubroot. Do not contaminate the area by using irrigation
water from sources that may be contaminated. If there is any doubt about soil
contamination, seedbed areas may be fumigated with = methyl bromide, Vapam,
Vorlex, or = chloropicrin according to the manufacturer's directions.
(Restricted-use pesticides are identified by a dagger, = )

(3) If a field is known to be infested, the soil pH should be
adjusted to pH 6.8 with ground limestone (CaC03). In addition to the limestone,
broadcast and incorporate at least 1,500 pounds per acre of hydrated lime
(calcium hydroxide Ca(OH)2) before planting. It is essential that hydrated
lime be added each year unless the soil pH exceeds 7.5. Regardless of how
finely ground, limestone alone will not control clubroot. Soil test recommendations
from Cornell will include liming requirements for crucifers as part of the
computer printout.

Although the way in which lime reduces infection by P. brassicae
is not understood, it is thought that control is enhanced when the calcium
concentration in the soil is very high. Because the use of nitrogen fertilizers
will reduce soil pH (nullifying the beneficial effects of liming) and because
some crops in rotation with crucifers are sensitive to heavy lime applications,
it is important to check the soil pH regularly.

(4) In addition to lime, PCNB (Terraclor 75% WP) can be used
in one of three ways:

(a) For growers who use transplant machines that apply water
around each plant as it is set out, the most efficient and economical method
is to apply PCN13 as a transplant solution at the rate of 3-6 lb per 100 gal
of solution and 1 /3-3/4 pt per plant.

(b) A more costly practice is to use 25-50 lb. per 13,000 linear
feet in a 12-in. band and incorporate to a depth of 4-6 in. before planting.

(c) For seedbed treatment, use 40-80 lb. per acre broadcast
and incorporate to a depth of 4-6 in. before planting.

The combination of hydrated lime and PCNB application has
provided excellent control. Do not use PCNB emulsion because it will cause
root burn.

(5) Always thoroughly clean farm equipment after working
in a field suspected of having the clubroot fungus.

(6) A rotation of at least 7 years out of susceptible crucifers
may be effective. For this rotation to be effective, make sure susceptible
weeds and plants mentioned under "Host Range" are excluded.

(7) Resistance: The first resistant cabbage variety Badger
Shipper from Wisconsin was developed by a chance cross of cabbage with kale.
Additional cabbage breeding lines that are promising have been released from
Oregon. Because of the existence of different clubroot races in many localities,
varieties claiming resistance need to be evaluated under New York conditions.
Always use agricultural chemicals exactly as stated on the label and follow
all safety and precautionary statements. The use of trade names in this publication
does not imply endorsement of the products named or criticism of similar ones
not mentioned.